Conductor powders

ABSTRACT

Nickel powders having an improved chemical stability are formed by reaction with an electroconductive coating such as a carbide, a silicide, or the like and, also, a reaction with sulfur.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for imparting an improvedcorrosion-resistant property to particles having metal surfaces. Inparticular, the invention relates to a means to achieve an improvedcombination of good chemical resistance and good electroconductivity onmetallic particles. The invention also relates to the novel productsformed by the aforesaid process and to novel conductive paintcompositions utilizing said products.

It has long been a problem to provide electrically-conductive particleswhich have good corrosion resistance. One approach to such particles isdescribed in U.S. Pat. Nos. 4,092,459 and 4,137,361 to Robert J.Deffeyes. Deffeyes discovered that carbiding the surface of, say, anickel powder provided a nickel powder of good conductivity and goodchemical resistance.

Takahasi in U.S. Pat. No. 3,904,448 have described a thin coating ofnitride on a metallic powder product. The coating offered good corrosionresistance but had relatively poor electroconductive properties. Thepresent inventor has discovered a way to further improve theparticle-treating processes of the general type described by suchinventors. Such particles find exceptional value in the formation ofcoating compositions for electroconductive, or electromagnetic-shieldingapplications.

SUMMARY OF THE INVENTION

It is a principle object of the invention to provide electroconductiveparticles with an improved combination of electroconductivity and goodchemical resistance.

Another object of the invention is to provide an improved process forthe purpose of making electrically-conductive particles having anexceptionally good combination of initial electroconductivity and goodresistance to decay of that electroconductive property.

A further object of the invention is to provide an improvedelectroconductive particle of nickel of the type wherein anelectrically-conductive carbon compound is present on the surface of theparticles.

A further object of the invention is to provide improvedelectroconductive coating compositions.

Other objects of the invention will be obvious to those skilled in theart on their reading of this disclosure.

The above objects have been substantially achieved by combining with acarbiding, siliciding, nitriding or other protective treatment, atreatment of metal particles with a carefully selected quantity ofsulfur compound. A synergistic effect is achieved whereby the presenceof the sulfur atoms on the surface of the particles aids to increase thecorrosion resistance of the particles, as measured in terms of loss ofelectrical conductivity, without interfering with good initialelectroconductivity.

In the preferred embodiment of the invention, the conductive powder isnickel, or iron, or cobalt. However, it is always an electroconductivemetal and the coatings, e.g. the carbide or silicide thereof are alsoelectrically conductive.

Among the metal powders which may be treated according to the inventionare nickel, cobalt, iron, manganese, chromium, molybdenum and the like,i.e. those metals of good electroconductive properties and which formelectrically conductive silicides, borides or nitrides or oxides and,most favorably, carbides.

It is desirable to treat the metal powder to be processed with areducing agent, for example, hydrogen gas, before the process of theinvention is carried out.

The particles of the invention are generally those having an averagediameter of about 0.5 to 40 microns, i.e. those having a high surfacearea per unit weight. The articles may be rod-shaped, flake-shaped orspherical.

The amount of sulfur used must be carefully controlled. Too much sulfurinhibits the desirable electroconductivity of the particles. Among theorganosulfur compounds which are useful are 1-dodecanethiol;dithiobisteropropionate and octadecyl 3-mercaptopropionate. Since theprocess is operated at elevated temperatures, care should be taken toavoid compounds which will distill off from the reaction medium takingsulfur with them.

The preferred reaction medium should be a sulfur-free oil. Care shouldbe taken in making this selection and an oil which has been acid washedis preferred. A mineral oil sold under the trademark Primol 355 by Exxonis one suitable liquid medium for carrying out the process of theinvention.

In the process of the invention, a predetermined quantity of sulfur willbe added to a liquid environment within which the conductive metalsurface to be coated will be subjected to a primary reaction gas, e.g.carbon monoxide which will form a carbide on the metal. Other primaryreactants can contribute silicon, nitride, etc. all as known in the artand disclosed in, e.g. U.S. Pat. Nos. 4,137,361 and 3,904,448.

The reactant is advantageously a gas which will contribute carbon to thecoating to be placed on the metal surface. For example, carbon monoxideor methane is a good source of carbon. However, it has been found thatthe corrosion resistance of, say, a carbided surface can be markedlyenhanced if a pre-determined amount of sulfur is added to the liquidmedium. For example, organosulfur compounds are believed to be ofparticular value when a mineral oil is used as the liquid reactionenvironment.

ILLUSTRATIVE EXAMPLES OF THE INVENTION

In this application there is shown and described preferred embodimentsof the invention and suggested various alternatives and modificationsthereof, but it is to be understood that these are not intended to beexhaustive and that other changes and modifications can be made withinthe scope of the invention. These suggestions herein are selected andincluded for purposes of illustration in order that others skilled inthe art will more fully understand the invention and the principlesthereof and will be able to modify it, each as may be best suited in thecondition of a particular case.

EXAMPLE 1

The following process was carried out in an agitated electrically-heatedreactor of nominal 35-gallon capacity.

The reactor is purged with nitrogen as the reactor is being heated. Thenone hundred lbs. of nickel flake, e.g. that sold under the tradedesignation Nickel UFNL by Novamet Inc. of Waldwick, N. J., is uniformlydispersed into a white mineral oil sold under the trade mark Primol 355by Exxon Corp. The final volume of the metal-in-oil slurry is about 25gallons.

To the slurry, as the sole source of sulfur, is mixed 0.8 lbs of1-dodecanethiol. The nitrogen purge is continued throughout these stepsand for 30 minutes thereafter. Then the reactor is sealed and thepressure is increased to 500 psig. A high-pressure source of nitrogen isused to achieve this pressure in the reactor over the slurry.Thereafter, the nitrogen is replaced with hydrogen gas and a small flowof hydrogen is maintained through the reactor at 500 psig. After thereactor reaches 400° F., (in this case a period of about 1 hour afterthe hydrogen atmosphere has been established), the slurry is agitatedfor a period of about four hours at 400° F. (All gases are bubbled intothe autoclave from the bottom)

Thereupon, the hydrogen gas is replaced with methane gas. The methanegas flow is continued for another four hours after which the heat isturned off and the methane gas flow is replaced with nitrogen.

The nitrogen purge is continued until the gas discharging from thereactor is no longer flammable. (It is convenient to burn the off-gas asa flare, and the off-gas may be considered to be non-flammable when onlythe pilot light is burning at the flare)

When the slurry cools to about 200° F., it is dumped through a heatexchanger to remove more heat and thence into a hold tank wherein thetreated metal product is allowed to settle out. Thereupon, the liquid isdecanted off, and the product is washed with perchloroethylene. Next thenickel is reslurried in clean solvent, filtered and dryed.

After being sieved through a 400 mesh sieve, the treated metal is readyfor use, e.g. as a corrosion-resistant material in the use ofelectroconductive paints and the like.

The nickel flake utilized is believed to be made by nickel carbonylreduction process and has an average particle size of about 1 micron.The finished flake averages about 0.1 micron thick and about 2-8 micronsin diameter. The product has a surface area of about 0.4 square metersper gram by B.E.T. analysis based on the absorbtion of nitrogen.

The powder is dispersed into a test formulation comprising 80% by weightof powder to, 20% by weight of acrylic binder, plus enough solvent toreduce the viscosity of the composition comprising the powder and theliquid vehicle to about 22 seconds on a No. 2 Zahn cup. This mixture isabout 50% solids. The resulting paint is sprayed with a Binks No. 19spray gun to form a coating of paint 2 mils thick on a 7 mil polyesterfilm. The initial conductivity of the paint is about 0.75 ohm persquare. Results as low as 0.5 ohms per square have been achieved.Resistivities of below 1.5 ohms per square are usually achieved andconsidered to be "in specification" although paints of higherresistivity, e.g. 10 ohms per square are very useful in manyapplications.

The painted strip of polyester is placed in an environmental chamber at160° F. and 50% relative humidity for 120 hours. Then the conductivityis measured again. Typically, the resistivity rises to 2.50 ohms persquare, but 5 ohms is suitable. When the same process is run without thesulfurizing, the material degrades faster as measured by the measured byelectroconductivity of the paint. For example, it will typically rise toabout 100 ohms per square. (If one uses only the sulfur step andeliminates the carbiding step, the same undesirable rise in resistivityoccurs)

EXAMPLES 2-3

Example 1 was repeated but, instead of using 1-dodeconethiol,dithiobisteropropionate and octadecyl 3-mercaptopropionate were used inExamples 2 and 3, respectively. These materials were used in 0.001548sulfur-to-nickel weight ratio. In each case an electrically-conductivematerial of superior corrosion resistance was obtained.

EXAMPLE 4

Example 1 is repeated using iron. A conductive, corrosion resistantiron-based powder is obtained.

EXAMPLE 5

Example 1 is repeated using cobalt. A conductive, corrosion-resistantcobalt powder is obtained.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which mightbe said to fall therebetween.

What is claimed is:
 1. An electroconductive paint product, containingelectroconductive metal particles in a liquid vehicle, said particlesbeing present in a quantity effective, upon solidification of saidpaint, to be in particle-to-particle contact with one another, andwherein said particles have a stabilized electroconductive surfacecomprising(a) a first reaction product of said electroconductive metalwith an electroconductive carbide, nitride, boride or silicide and,also, (b) an effective amount of second reaction product of saidelectroconductive metal with sulfur to form means to improve thestability of the electroconductivity of a coating formed of said paintto moisture and heat.
 2. A paint as defined in claim 1 wherein saidfirst reaction product is a carbide and wherein said metal is nickel. 3.A paint product as defined in claim 1 wherein said metal is iron, nickelor cobalt.
 4. A solidified paint product comprising an effectivequantity of particles having a stabilizing electroconductive surface asdefined in claim 1, 2 or 3 and further characterized by an initialresistivity of less than 10 ohms per square when said particles areloaded into a 2-mil coating of said paint at 80% by weight.
 5. A paintas defined in claims 1, 2, or 3 wherein the stabilized powder has anaverage particle diameter of from 0.5 to 40 microns and is characterizedby the ability to impart a resistivity of 1.5 ohms per square or lesswhen distributed at 80% by weight leading, in a 2-mil thick coating ofpolyacrylate binder.